Electrolysis

• Electrolysis


  The process where an electric current is passed through an electrolyte, causing the ions to move to the electrodes and form pure elements
• Electrolyte


  • A liquid or solution that contains ions and can conduct electricity
• Electrolysis


  1. Ions move to the electrodes

  2. Ions form pure elements at the electrodes
• Anode


  • Positive electrode
• Cathode


  • Negative electrode
• Molten ionic compounds


  Solid ionic compounds that can conduct electricity when molten or dissolved, as the ions are then free to move
• Electrolysis of molten ionic compounds

  1. Positive metal ions attracted to cathode, gain electrons to form pure metal

  2. Negative non-metal ions attracted to anode, lose electrons to form pure non-metal
• Molten ionic compound


  • Molten sodium chloride, NaCl
• Electrolysis of molten sodium chloride

  1. Sodium formed at cathode

  2. Chlorine formed at anode
• Half equations


  Equations showing the reactions at the cathode and anode during electrolysis
• Electrolysis of aqueous solutions

  1. Requires less energy than melting ionic compounds

  2. Pure elements not always produced, as water can also undergo ionisation
• Water ionisation


  H2O(l) → H+(aq) + OH-(aq)
• Products at cathode in electrolysis of solutions

  1. If metal more reactive than hydrogen, hydrogen gas formed

  2. If metal less reactive than hydrogen, metal deposited
• Products at anode in electrolysis of solutions

  1. If non-metal contains oxygen, oxygen gas formed

  2. If non-metal is a halogen, halogen gas formed
• The OH-(aq) ions formed from water ionisation are attracted to the anode
• The H+(aq) ions from water ionisation are attracted to the cathode and react with it to form hydrogen gas
• The metal ions remain in solution after electrolysis
• Electrolysis of aluminium oxide

  1. Aluminium oxide is mixed with cryolite

  2. Mixture is heated until molten

  3. Molten mixture undergoes electrolysis
• Electrolysis


  Used to extract metals from their ionic compounds
• Electrolysis


  Used if the metal is more reactive than carbon
• Aluminium


  Extracted from aluminium oxide by electrolysis
• Electrolysis of aluminium


  1. Aluminium oxide is mixed with cryolite to lower the melting point

  2. Mixture is heated until molten

  3. Molten mixture undergoes electrolysis
• Cathode


  • Pure aluminium is formed
• Anode


  • Oxygen is formed
• In the electrolysis of aluminium, the anode is made of graphite
• The graphite reacts with the oxygen to form carbon dioxide and so slowly wears away, needing to be replaced frequently
• Lining


  • Negative electrode made from carbon
• Positive electrode


  • Made from carbon
• Oxygen is produced at the positive electrode and reacts with the carbon of the electrode to form carbon dioxide gas
• Molten aluminium is produced and removed
• Key terms


  • anode
  • cathode
  • electrolysis
  • cryolite
  • electrolyte
  • electrode
  • reactivity
• Reactivity series


  • Potassium
  • Sodium
  • Calcium
  • Magnesium
  • Aluminium
  • Carbon
  • Zinc
  • Iron
  • Tin
  • Lead
  • Copper
  • Silver
  • Gold
  • Platinum
• The H2 gas is formed at the cathode
• The O2 gas is formed at the anode
• The more reactive a metal is


  The more easily it is oxidised
• Metals higher in the reactivity series are more reactive
• What is electrolysis?


  process of using electricity to extract elements from a compound
• What is the name of the positive electrode?

  anode
• What is the name of the negative electrode?

  cathode
• What is an electrolyte?

  liquid or solution that contains ions and so can conduct electricity